The present invention relates to an improved die for use in the manufacture of self-drilling screws, and, in particular, to a die that produces a forged blank having an endchip that may be removed in one piece during the thread-rolling process.
Screws and fasteners having the capability for drilling a pilot hole and forming threads in the workpiece without the need to predrill are well known. Such screws and the dies by which they are made are illustrated in U.S. Pat. Nos. 3,395,603; 3,517,542; and 4,836,730.
The procedure used to manufacture self-drilling screws according to these patents is to impression forge a blank using a pair of matching dies to form the point and flute section of the screw. The forged blank is then thread-rolled to form the threaded section. Dies used in the forging process generally consist of a rectangular body having a work face with a conical recess therein. The work face of each die is orientated so as to align in face to face abutment with the work face of the matching die. In this way, the conical recess of each die act in conjunction to define a space for a screw blank to be inserted and forged therein. Jigs are often used in the forging process to secure the dies in place, as disclosed in my prior patent U.S. Pat. No. 5,014,540.
Although many of the fasteners disclosed in the prior art have had commercial success, there have been areas recognized that are subject to improvement. For instance, it has been found that during the forging process described, some of the blank material flows outward of the forging cavity forming an endchip. The endchip consists of a flash of material that is left around and just below the head of the self-drilling screw.
In conventional die designs the endchip created consists of a series of brittle fragments that during the thread-rolling process fracture in multiple pieces leaving a portion of the endchip in the flute section of the screw. This results in a screw having diminished drilling performance. As a result, the screw may be difficult to install or can fail completely during installation.
The endchips created by traditional dies can also create problems in post forming processes. If the screw is heat treated the endchips can fall off and clog the furnace; or if the screw is plated the endchip can dislodge during the plating process and attach to recesses in the head portion of the screw causing problems in the installation of the screw.
It is, therefore, the object of the present invention to provide an improved die design for use in the manufacture of self tapping screws. Specifically, it is the object of the present invention to provide a die design that produces a workpiece having an endchip that can be rolled off in one piece during the thread-rolling process, thereby producing a screw having improved surface finish and drilling performance. An additional object is to provide a die that will reduce costs and downtime associated with post-forming processes.
The foregoing objects and numerous advantages of the present invention will be apparent from the following disclosure.